1. Field of the Invention
This invention relates to an improved PTC electric heating element assembly for heating a container. The PTC electric heating element assembly comprises a heating element of a ceramic material with positive temperature coefficient, two insulating elements with different thickness, and four metal sheets, which are arranged in a stack by passing through a headless bolt welded on the bottom of a container to firmly join them together. The opposite sides of the heating element are contacted by metal sheets having electrodes which are connected to electric wires connected to electric power source.
2. Description of the Prior Art
In conventional electrical heating appliances, most of the electric heating elements are electric heating wires, which serve as electric heating source. However, the electric heating wires have several disadvantages such as the heating efficiency which is unsatisfactory, causing the electric heating wires to oxidize so that the life is shortened, and making it necessary to add a temperature controller for controlling the heating temperature. As is well known, the conversion efficiency of electric heating from electric heating source is about 40%, and the resistance in the electric heating wire is kept steady the electric heating wire is heated so that it continuously consumes electricity without increasing the conversion efficiency of electric heating. Besides, the heating temperature increases at a gradual rate (i.e. it takes long time to heat the electric wire to a certain temperature) since the electric heating wire needs to be preheated during the heating procedure. In order to overcome these disadvantages, PTC electric heating elements are increasingly used for heating purpose. Although the PTC electric heating elements do not have the above-mentioned disadvantages, the prerequisite for practical usage of the PTC heater demands assembly of the PTC electric heating member in a suitable manner to attain good electrical insulation and thermal conductivity.
For example, U.S. Pat. No. 4,324,974 discloses a heating element assembly with a PTC electric heating element for insertion in a recess of a heater, wherein two opposite sides of the heating element are contacted by planar electrodes and give off the heat produced through the electrodes and planar sheets of insulating material covering the same to two heat-absorbing surfaces of the heater. In particular, the sheets of insulating material are coated on their side facing the heating element over a large area with solder and are soldered to the electrodes. U.S. Pat. No. 4,327,282 shows an electrical resistance heating element comprising at least one PTC heating conductor arranged in sandwich form between contact plates. The components are held together elastically by a series of U-shaped clips or U-shaped strips holding the edges of the contact plates. U.S. Pat. No. 4,730,103 shows a compact resistor heater device comprising a PTC resistor body, electrical lead-in wires being bonded to each side of the resistor body, and a heat sink used for disposing the resistor body and the wires thereon, being encapsulated with a thermally conductive sheet.
Unfortunately, the drawback of many of the aforementioned electric heating assemblies is that they either lack the ability to change for different electric heating demands or, as in the case of the heating element disclosed in U.S. Pat. No. 4,327,282, they will be loose for a certain time due to elastic fatigue of the securing means.